In applications using pulsed laser beams it is often advantageous to have some measure of control on the temporal shape of the pulse. The resultant pulse may be temporally compressed, expanded, or structurally synthesized to activate some physical process that would have otherwise been difficult or impossible. An example application is the selective photoexcitation of the particles of plural isotope types which need to be separated. A discussion of this is found in U.S. Pat. No. 4,288,691 to J. A. Horton (issued Sep. 8. 1981) and U.S. Pat. No. 4,283,116 to J. A Weis (issued Aug. 11, 1981). Other application areas include laser material processing high energy physics, coherent spectroscopy, and optical signal processing.
A widely used and researched facet of laser pulse shaping involves the modification of a laser pulse using spectral filtering. Such filtering is obtained with prisms and/or gratings in optical systems which generally separate the constituent wavelength components of a laser pulse and modify them such that the optically recombined and modified wavelength components produces a pulse with a generally different shape than it had originally. Laser pulses that are narrow in time to begin with have wide bandwidths which allow the use of spectrally dispersive optical elements for pulse shaping. However, the utility of this method is diminished for spectrally narrow laser pulses as discussed by Weiner et al. in "High-resolution femtosecond pulse shaping," J. Opt. Soc. Am. B., Vol. 5, August, 1988, p. 1563, and Reitze et al., "Shaping of wide bandwidth 20 femtosecond optical pulses," Appl. Phys. Lett., Vol. 61, Sept., 1992, p. 1260. Optical energy losses are another disadvantage of spectrally dispersive optical systems used for active pulse shaping.
The difficulty of the above method for shaping laser pulses that are not originally narrow in frequency has precipitated other ways of producing shaped laser pulses. A general method splits a single laser pulse into a sequence of time-delayed pulses and recombines them in some way to form a pulse envelope of desired shape, usually longer than the original pulse. One simple method uses a passive pulse stacking scheme disclosed in U.S. Pat. No. 3,879,109 to C. F. Thomas (issued Apr. 22, 1975). No active mechanism is required and the system uses two sets of partially transmissive parallel mirrors spaced apart and with differing reflectivities, to obtain an exponential envelope. Any change in system parameters usually requires replacement of one or more mirrors and realignment of all. This method does allow for pulse compression.
U.S. Pat. No. 4,059,759 to R. C. Harney (issued Nov. 22, 1977) uses a magneto-optical (Faraday) polarization rotator in an optical system to generate a composite pulse or sequence of pulses with a pulse envelope of controllable temporal variation. One embodiment of this patent uses the Faraday rotator to switch during the passage of an optical pulse in order to segment the pulse into two portions for eventual recombining. However, there is not an allowance for multiple segmentation which limits the temporal resolution of the modified laser pulse. The system is limited by the relatively slow electrical activation of the Faraday cell since it requires large magnetic fields (hundreds of kGauss) supplied by inductive coils.
U.S. Pat. No. 4,288,691 to J. A. Horton (issued Sep. 8, 1981) discloses the use of a system of Pockel cells and polarizing beam splitters to temporally segment a laser pulse into two or more sections. The sections are then superimposed after a delay to create some desired shape. This scheme can produce pulse compression by a factor of approximately two for one Pockel cell. In order to segment a laser pulse into greater than two sections, a matrix of Pockel cells and polarizers are necessary. Also, the temporal narrowness of the segmentation depends on the limited speed of the Pockel cells which in turn limits the feature resolution of the synthesized pulse shape.
Another method to produce arbitrarily shaped laser pulses uses electro-optic beam deflection across an aperture or spatial filter. Such a scheme was reported by C. L. M. Ireland in the paper "Some design considerations and application of a fast crystal deflector," Proc. of the Fourth National Quantum Electronics Conference, 1979, p. 87. Here an experiment is described where a laser beam is rapidly scanned across a slit using electro-optic beam deflection to produce a laser pulse on the output of the slit with a temporal width equal to the slit width divided by the linear scan velocity. Similar arrangements using electro-optic beam deflection with apertures is reported by T. Kobayashi et al. in "Generation of arbitrarily shaped optical pulses in the sub-nanosecond to picosecond region using a fast electro-optic deflector," IEEE J. Quantum Electronics, Vol. QE-16, Feb., 1980, p. 132. In this paper one variation beyond the system demonstrated by C. L. M. Ireland uses an amplitude spatial filter in place of a slit which simply gives a transmission coefficient to light that varies with spatial position. A patterned filter of varying opacity is scanned using an electro-optic deflector to amplitude modulate a continuous wave (CW) or pulsed laser beam. This scheme is limited practically since the amplitude modulation is being accomplished at the great expense of lost optical energy.
A second variation discussed in the same paper by T. Kobayashi et al. uses in place of the amplitude spatial filter what the authors refer to as a phase-responsive spatial filter. Examples given of phase-response spatial filters are a glass wedge, a simple lens, and a biprism or smoothed biprism. A glass wedge in front of the electro-optic beam deflector modifies the temporal position of an entire laser pulse, while the biprism creates two pulses, each half the amplitude of the original. A simple lens that is electro-optically scanned can temporally broaden the original pulse. The advantage of these phase-responsive spatial filters is that optical energy is largely conserved unlike the amplitude spatial filters. However, no configuration using these or other optical elements is proposed which can facilitate optical pulse compression. Furthermore, the parameters that allow feature control of the pulse shaping are few and no scheme is reported that allows flexible synthesis of arbitrarily shaped optical pulses while largely conserving the energy of the original pulse.
A third variation mentioned in the same paper by T. Kobayashi et al. uses an electro-optic beam deflector and diffraction grating which can facilitate pulse compression. A short laser pulse is first produced by electro-optically scanning across an amplitude spatial filter as discussed earlier. This pulse is then reflected off a diffraction grating which acts like a multiplexer in the optical frequency domain to recombine the constituent optical frequency components to produce a temporally compressed pulse. However, this scheme again uses the ability to spectrally filter which first requires generation of a short laser pulse. This generated pulse must be broad enough in bandwidth to give a suitable magnitude of angular dispersion from the grating and would not work for an arbitrary input laser pulse.